Conventionally, a catalyst supported on a honeycomb structure made of cordierite or silicon carbide is used for treatment of harmful substances in an exhaust gas discharged from a motor vehicle engine (see, Patent Document 1). Such a honeycomb structure generally includes a pillar shape honeycomb structure portion that has partition walls serving as flow paths for an exhaust gas and defines a plurality of cells extending from one bottom surface to the other bottom surface.
When treating the exhaust gas with the catalyst supported on the honeycomb structure, a temperature of the catalyst should be increased to a predetermined temperature. However, there has conventionally been a issue that since the temperature of the catalyst is low at the start of the engine, the exhaust gas is not sufficiently purified. Therefore, there has been proposed a system called an electrically-heated catalyst (EHC) which increases a temperature of a catalyst supported on a honeycomb structure comprised of conductive ceramics to an activation temperature before or during the start of an engine by providing electrodes to the honeycomb structure and causing the honeycomb structure itself to generate heat by energization. In the EHC, it is desired to allow the catalyst to satisfactorily function by causing the catalyst to uniformly generate heat (with no deviation in a heat generation distribution), without damaging electric systems in a motor vehicle.
In Patent Document 2 proposes a honeycomb structure that is a catalyst carrier, also functions as a heater by applying a voltage, and can suppress a deviation of a temperature distribution when the voltage is applied. Specifically, it proposes that a side surface of the pillar shape honeycomb structure is provided with a pair of electrode portions in a band-like shape extending in a cell extending direction of the honeycomb structure portion, and in a cross section perpendicular to the cell extending direction, one of the electrode portions of the pair of electrode portions is arranged on an opposite side to the other of the electrode portions in the pair of electrode portions sandwiching a center of the honeycomb structure portion, thereby suppressing the deviation in the temperature distribution when the voltage is applied.
Further, Patent Document 3 discloses an approach to scatter portions where heat generation is concentrated in a honeycomb structure, thereby suppressing a local temperature rise of the honeycomb structure and causing the honeycomb structure to generate heat more uniformly. Specifically, Patent Document 3 discloses a honeycomb structure in which each of a pair of electrode portions is formed of a laminate of two or more electrode bodies, and a center angle of the electrode body closest to an outer peripheral wall of the honeycomb structure portion is larger than the central angle of the other electrode bodies.